Recently a passenger protection device such as an airbag and a seatbelt pretensioner is mounted in many vehicles. A passenger protection system having the passenger protection device includes, as shown in FIG. 8, front sensor units 11a, 11b mounted at both front sides of a vehicle 10, safing sensor units 13a, 13b mounted at an assistant seat and rear seats, and a plurality of sensor units 15a, 15b, 15c 15d and 16a, 16b, 16c, 16d mounted at both sides of the vehicle 10. These sensor units are connected to an ECU 18 for an airbag to form a communication device. Each sensor unit 11a, 11b, 13a, 13b, 15a to 15d and 16a to 16d detects acceleration and activates an airbag, which is not shown, in response to detection of acceleration. Here, each sensor unit 11a, 11b, 13a, 13b, 15a to 15d and 16a to 16d is formed in an integrated circuit (IC) chip.
In this communication device, as representatively shown in FIG. 9, the sensor units 15a to 15d and 16a to 16d at both sides have respective bus switches 26a to 26d internally and are connected to the ECU 18 through buses. Further, when power supply of the vehicle 10 is turned on, the sensor units are set with addresses and initialized to turn on the bus switches 26a to 26d in order from the sensor unit closer to the ECU 18. That is, after setting the address in the first sensor unit 15a, which is closest to the ECU 18, the bus switch 26a is turned on to connect the ECU 18 to the second sensor unit 15b. After setting the address in the second sensor unit 15b from the ECU 18, the bus switch 26b is turned on to connect the third sensor unit 15c to the ECU 18. Further, after setting the address in the third sensor unit 15c, the bus switch 26c is turned on to connect the fourth sensor unit 15d to the ECU 18. The initialization is thus performed. Each of the sensor units 15a to 15d is configured to return a response to the ECU 18 after the address setting.
In case that the sensor units 15a to 15d and 16a to 16d on both sides are connected to the ECU 18 by buses, it is necessary to provide the bus switches 26a to 26d inside the sensor units 15a to 15d and 16a to 16d formed in IC chips, respectively. As a result, the chip size becomes large. Since each bus switch has impedance, the impedances of the sensor units 15a to 15d and 16a to 16d cause voltage drops when the plurality of the sensor units 15a to 15d are bus-connected. As a result, the voltage drops at the sensor units 15d, 16d of the end stage become large. In addition, the bus switch becomes a noise source, which generates noise, when impedances at the power supply side and the ground side of the bus switches do not match.
To solve this problem, according to a communication device disclosed in JP 2010-137840A (US 2010/0121534A1), bus connection by bus switches is not performed. According to this configuration, as shown in FIG. 10, sensor units 15a1 to 15d1 are connected to an ECU 181 in a daisy chain form without using bus switches. In this configuration, the sensor unit 15d1 at the last stage is set with an address 0001 at time t1 first for transition to a sleep mode. Since no current flows to the fourth sensor unit 15d1, which is in the sleep mode, the third sensor unit 15c becomes the last stage unit and ready for being set with an address. Similarly as described above, the third sensor unit 15c1 is set with an address 0010 at time t2 for transition to the sleep mode. Further, as indicated at time t3 and t4, addresses 0100 and 1000 are set in the sensor units in sequence toward the ECU 181 and the sensor units are rendered to be in the sleep mode in sequence. Thus, the initialization is completed. After completion of the initialization, the ECU 181 transmits a sleep mode cancellation command thereby to cause the sensor units 15a1 to 15d1 to return to respective normal operation mode. Thus bus switches are not necessitated.
It is assumed in this communication device for a passenger protection system that a connection line between the first sensor unit 15a1 and the second sensor unit 15b1 is disconnected (open-failure) as indicated by a mark X in FIG. 11, or the third sensor unit 15c1 fails (sensor failure) as indicated by a mark X in FIG. 12 although the current path from the ECU 181 to the sensor unit 15d1 at the last stage is normal. In this case, as described later, it is not possible to determine whether it is the open-failure or the sensor-failure. It is not possible either to specify between which sensor units the open-failure arose or which sensor unit has the sensor-failure.
That is, in case of determining the open-failure or the sensor-failure, ECU 181 is configured to determine whether the address setting at the time of initialization has been performed normally. In FIG. 11, since the first sensor unit 15a1 is the last stage unit, the address 0001 is set in the first sensor unit 15a1 first. If the first sensor unit 15a1 is rendered to be in the sleep mode, thereafter no other sensor unit is connected to the ECU 181. As a result, the address setting is performed only once. The ECU 181 determines that the open-failure or the sensor-failure is present based on that the address setting is not performed appropriately.
In FIG. 12, when the sensor unit 15d1 at the last stage is set with the address 0001 and then rendered to be in the sleep mode, the second sensor unit 15b1 is set with the address 0010 because of the sensor-failure of the third sensor unit 15c1. Then the first sensor unit 15a1 is set with the address 0100. In this case, only three addresses are set although four addresses should be set. The ECU 181 determines that the open-failure or the sensor-failure is present based on that the address setting has not been performed normally. In either case, it is not possible to determine whether the failure is the open-failure or the sensor-failure. It is not possible either to specify between which sensor units the open-failure arose or which sensor unit has the sensor-failure.